The present invention relates to the continuous production of thermally insulated conduits and tubes.
The German printed patent application No. 2,525,561 discloses a method of thermally insulating a metal tube by means of a crosslinked and foamed plastic jacket. A thermoplastic powder or granulate is mixed with a blowing, expanding or foaming agent as well as with crosslinking agents and is heated in a blender or an extruder. A fast rotating shear head increases the temperature above the reaction temperatures of the crosslinking and blowing additives so that the plastic is crosslinked at first, followed by expanding into a cellular configuration and texture.
The method as described was used particularly on an experimental basis, to melt a mixture of high density polyethylene and low density polyethylene, includes at least 50% of the high density component, melting to take place in the presence of a peroxide and in an extruder so that a shear head can be charged with a homogenic blend. The pressure of the flowing or fluid mass was measured right in front of the entrance to the head and was found to vary to a considerable extent. It is believed that these pressure variations originate in pulsations of and in an inhomogenic melt upstream from this shear head exit and containing molten substance mixed with not yet or partially molten granules.
The melt is believed to be inhomogenic due to the different melting temperatures of high and low density plastic. Particularly the melting point of high density polyethylene is rather high (about 130.degree.) while peroxide begins to become reactive towards formation of radicals at temperatures not much above 130.degree. C., so that the polymers begin to crosslink. Crosslinking begins prematurely in the sense that molecules in the liquid phase crosslink while high density granules are still not melted. In order to prevent such premature crosslinking prior to complete melting of the polyethylene blend, one must operate in a very limited temperature range. Low density polyethylene by itself melts already at about 110.degree. C. so that the permissible operating range is significantly larger. The problem above arises from a higher melting point of high density polyethylene. It is, therefor, not surprising that the employment of high density polyethylene has been more or less avoided. Large scale processing makes it difficult to impossible to maintain a sufficiently narrow temperature range above 130.degree. C. but below the onset of crosslinking. Nevertheless, low density polyethylene by itself is fairly weak mechanically so that some higher density polyethylene should be added to strengthen the foamed coating. This is particularly true for sphereodized copper tubes, jacketed in foam plastic. That plastic jacket should be strong but not so stiff so as to resist bending of the tube for installation.